Vibration-induced size separation in granular media: The convection connection

We have investigated the rise of a single large glass bead through a vibrated cylindrical column of smaller particles and found that vibration-induced size separation in this geometry arises from convective processes rather than from local rearrangements as had been proposed previously. A convection cycle, rising in the middle and dropping in a thin stream along the walls of the cell, is responsible for all particle movement. Particles larger than the width of the thin downward convection zone are carried to the top of the column and then trapped, resulting in size segregation. For a variety of accelerations the position of the rising particle can be scaled onto a single curve.